1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an inkjet recording method, to record images on a recording medium by jetting inks.
2. Description of Related Art
There has been known an inkjet recording apparatus which records images by jetting inks from nozzles on recording heads, as a recording apparatus printable on a recording medium, such as plain paper or the like.
Recently, in an inkjet recording apparatus, while it has been made efforts to achieve higher quality of images by making the density of nozzles on a recording head higher, the load of drive circuits for recording heads has been reduced by driving nozzles in each nozzle line on the recording head at different timings to reduce the number of synchronized nozzles.
As an inkjet recording apparatus in which nozzles in a nozzle line are driven at different timings, there have been known an inkjet recording apparatus in which the so-called staggered arrangement of nozzles are driven with a plurality of drive phases (refer to, for examples, JP-Tokukai-2002-137388A, JP-Tokukai-2003-326687A and JP-Tokukai-sho-59-33117A), and an inkjet recording apparatus that employs the so-called multi-pass recording system (refer to, for example, Japanese Patent 3441868). Here, the staggered arrangement is an arrangement that, in a nozzle line having a plurality of nozzles arranged in the conveying direction of a recording medium, nozzle positions are displaced in a scanning direction for every drive phase. The multi-pass recording is a recording system that a serial type recording head scans one same area on a recording medium by plural times to complete an image recording on the area.
In the recording head of the inkjet recording apparatus having staggered arrangement of nozzles, for instance, the nozzles are driven with 3-phase drive in order of phase 1, phase 2 and phase 3 for every 3 nozzles arranged in the conveying direction. That is, as shown in FIG. 13A, nozzles 30a, 30b and 30c corresponding to phase 1, phase 2 and phase 3, respectively, are so controlled that their phases are switched by respective strobe pulses STB 1 to STB 3. In this inkjet recording apparatus, the nozzle position displacement can be compensated by 3 phase switchings while the recording head moves by one pixel, and thus dots can be recorded in a straight line. In FIG. 13A, the strobe pulse STB 1 switches the phase of the nozzle 30a, STB 2 the nozzle 30b, and STB 3 the nozzle 30c. 
With use of a serial type recording head in which the above-described recording head is mounted on a carriage, each phase has to be switched while the recording head moves by one pixel for recording dots in a straight line, so that scanning speed of the carriage is limited by the number of drive phases for nozzles on the recording head. That is, the increased number of drive phases requires the increased number of switching of strobe pulses, which causes a strobe pulse width to be relatively narrower and the carriage speed to be reduced at the rate.
The scanning speed of the carriage is also limited by a staggered pitch p between nozzles. That is, because one pixel has to be recorded in a time t1 during which a nozzle moves by the staggered pitch p, a time t2 necessary for jetting ink for one pixel is not more than the time t1 (=staggered pitch p/scanning speed V), as shown in the following expression (1). Therefore, the upper limit of the scanning speed V is, as shown in the following expression (2), a value of the staggered pitch p divided by the time t2 necessary for jetting ink for one pixel. From this relationship, in order to get higher scanning speed, it may be a solution to make the staggered-pitch larger, but larger staggered pitch makes the size of the recording head larger, and requires new development of manufacturing technology.t2≦t1 (=p/V)  (1)V≦p/t2  (2)
As described above, an inkjet recording apparatus having staggered nozzles with multi-phase drive is limited in the scanning speed and cannot record images at higher speed.
On the other hand, as a recording head in an inkjet recording apparatus using the multi-pass recording system, there may be used such a head that adopts the so-called multi-phase drive method, for example, the same 3-phase drive as that of the head described above, in which, as shown in FIG. 13B, drive phases of nozzles 30a, 30b and 30c, corresponding to phase 1, phase 2 and phase 3 are controlled so as to be switched by strobe pulses STB 1 to STB 3, respectively. In this type of inkjet recording apparatus, pixels on one same line, which should originally be recorded by one same nozzle, are divided into plural sections and each section is recorded by mutually different nozzles. With this method, even if there is found misalignment of nozzles or ink jetting failure in some nozzles, these irregularities could be made averaged and could be perceived as unnoticeable dot displacement and the like. This type of inkjet recording apparatus is different from the inkjet recording apparatus having staggered nozzles, and can achieve higher image recording speed to the extent that the scanning'speed is not limited by the number of nozzle-drive phases and the staggered pitch.
In the inkjet recording apparatus using the multi-pass recording system as described above, let it be assumed that a plurality of nozzle lines are arranged on a carriage in a scanning direction, such as in the case as shown in FIGS. 14A and 14B, for example, that 4 recording heads are mounted on the carriage for jetting Y, M, C and K color inks, and that the number of pixels corresponding to the distance between nozzle lines is not equal to a multiple of the number of drive phases. With this structure, if nozzle lines are driven by the same phase at their drive timings, the relationship between the positions of a nozzle line in the scanning direction and the phases of the nozzle line differs from each other among the nozzle lines. Accordingly, relative positional relationship among the dots formed by the nozzle lines cannot be represented correctly, so that image quality of thin lines or characters is sometimes reduced.